Reducing energy usage in engineered systems is a primary concern because of the high cost and environmental impact of generating, transmitting and storing power. Lightweight components require significantly less energy to set them in motion. A component made from a polymer matrix composite (PMC) often weighs significantly less than the same component made from a metal or ceramic. While this benefit is commonly exploited in near ambient applications, high heat or cold applications, such as high-speed flight, vehicle components and structural support for batteries, fuel cells and other electronics are primarily handled by metals or high cost ceramic and metal matrix composites.
High performance fiber-reinforced PMCs have found widespread applicability in the automotive, aerospace and marine industries due to their low weight, excellent structural performance and manufacturability. PMCs provide outstanding benefits to structural systems at room temperature because of their high strength, high stiffness and low density. However, at high temperatures (ca. 100-300° C.), the structural capability of a PMC is greatly reduced, leading to the potential for material failure. This limitation is because the stiffness of polymeric materials, such as epoxy, polyethylene, and other plastics, is greatly reduced as the temperature of the material is raised. In addition, many plastics used as matrix materials are flammable, leading to additional dangers to using composites in places where fire is a possibility. While high temperature, non-flammable plastics exist, they almost universally suffer from high costs or difficult processing.
The service temperatures for a fiber-reinforced polymer matrix composites are severely limited by relatively low glass transition temperatures possessed by typical matrix materials, such as epoxy (ca. <200° C.), vinyl ester (ca. <150° C.) and bimaleimides and polyimides (ca. <250° C.). Above the glass transition temperature, the structural performance of a PMC is greatly reduced as a result of matrix softening. In addition, damage may form in PMCs after only a short exposure to high temperature, including delamination, matrix cracking, fiber-matrix debonding, combustion and fire. Accordingly, application of PMCs in high heat conditions requires a method for thermal regulation.
Active cooling through microfluidic channels has been utilized in electronics, fuel cells, high power batteries, micro-electro-mechanical systems (MEMS) and spacecraft systems. Several studies have examined the use of active cooling, where an external pump circulates liquid through internal micro-channels within a composite to remove heat. Results indicate that networks featuring branching and complex channel pathways are best situated to maximize the effectiveness of a cooling system. See e.g., Soghrati, S., P. R. Thakre, S. R. White, N. R. Sottos, and P. H. Geubelle (2012) “Computational modeling and design of actively-cooled microvascular materials,” Int. J. Heat Mass Transf, 55(19-20):5309-21; Soghrati, S., A. R. Najafi, J. H. Lin, K. M. Hughes, S. R. White, N. R. Sottos, and P. H. Geubelle (2013) “Computational analysis of actively-cooled 3D woven microvascular composites using a stabilized interface-enriched generalized finite element method,” Int. J. Heat Mass Transf. 65:153-64; Kozola, B. D., L. A. Shipton, V. K. Natrajan, K. T. Christensen, and S. R. White (2010) “Characterization of Active Cooling and Flow Distribution in Microvascular Polymers,” J. Intell. Mater. Syst. Struct. 21(12):1147-56; and Phillips, D. M., M. R. Pierce, and J. W. Baur (2011) “Mechanical and thermal analysis of microvascular networks in structural composite panels,” Compos. Part A: Appl. Sci. Manuf. 42(11):1609-19.
These studies have demonstrated active cooling as an effective method for enabling PMC service where high thermal loads are present. However, active cooling requires an externally powered pumping system and a method for removing heat from the fluid before it is re-circulated. The high power requirement for pumping liquid through the micro-channels can negate the energy savings gained from switching to a lightweight PMC. In addition, no methods currently exist for autonomic adaptive control (e.g. to automatically control flow rate in each channel) of the cooling, which means that conventional cooling systems can waste energy by pumping coolant to areas that are not being heated and/or pumping coolant when there is no need for it (e.g., when a low thermal load is present). Accordingly, none of the conventional systems are suitable for cooling large areas of a PMC without the need for external powering and/or controls.
Accordingly, there is a need for a system that can autonomously cool a structural material, allowing it to be used in places where it is subjected to both high heat and high structural loads. Such a system could replace heavy metal and/or ceramic materials and reduce system weight.
In this patent, we describe a novel cooling system and cooling method for materials, such as structural composites. The system is autonomic, self-powered and adaptable to changing thermal conditions without external sensing, control or powering.